In our everyday life, macroporous and microporous materials mostly made of petroleum based polymers are used in various forms and compositions. Examples of these are insulation in buildings and airplanes, and polymeric foams for packaging. Foams for this type of use have to be stable, light and easy to manufacture.
Due to the increased awareness of the need to use renewable materials, it is highly motivated to replace petroleum based polymers with polymers from renewable resources. Cellulose has a special potential, as the most abundant renewable natural polymers on earth with its crystalline structure and the availability of methods for preparing large volumes on an industrial scale. Cellulose chains with β-(1-4)-D-glucopyranose repeating units are packed into long nanofibrils in the plant, with cross-sectional dimension of 5-30 nm depending on the plant source. The parallel organization of the cellulose chains, held together by hydrogen bonds and organized in sheets, gives a crystal structure with a Young's modulus of approximately 130 GPa. These crystal domains are the reason why native cellulose I has such a high modulus and strength and it is interesting to consider these nanofibrils as being part of replacement material for petroleum based structures. Nanofibrils from cellulose have opened a new field as construction units for nanoscale materials engineering.
For more than a century, colloidal particles have been used to stabilize high energy interfaces in so-called Pickering emulsions. But it is only recently that this concept has been exploited for the preparation of ultra-stable wet foams and the preservation of these structures in a dry state to maintain porous materials. When particles are partially lyophobic or hydrophobic, they attach to the gas-liquid interface. It occurs because it is energetically favorable for particles to attach at the gas-liquid interface and replace part of the high energy solid-liquid area by a low energy solid-gas area. Preferably the particles should attach to the interface with a contact angle of approximately 90°. This is ultimately determined by the balance between the gas-liquid, gas-solid and solid-liquid interfacial tensions. In contrast to surfactants, particles tend to adsorb strongly at interfaces due to the high adsorption energy. This is the reason why particle-stabilized foams exhibits an outstanding stability compared to surfactant-based systems. Coalescence is hindered by the steric repulsion from the attached particles and additionally, the particles form a layer at the interface that strongly resists the shrinkage and expansion of bubbles, minimizing Ostwald ripening and creating long lasting stable foams. This further discussed in Studart, Gonzenbach, Tervoort and Gauckler, J. Am. Ceram. Soc., 2006, 89, 1771-1789.
U.S. Pat. No. 3,311,115 discloses a cigarette filter made of a dry porous cellulose, with the density of approximately 0.0008 g/cm3 (should be about 0.05 US pound/foot3). EP1960097B1 disclose methods on how to form stable foams from particles, but nothing is taught how to make foam from cellulose nanofibrils.
In this respect, modified cellulose nanofibrils are highly interesting for preparing highly porous renewable materials. However, cracks are usually formed in the material when wet foams are dried by freeze-drying. Thus there is a need for improved methods where wet foams can be dried so that the porous structure is maintained in the dry state.
It is also a need for foams based on a renewable material, such as cellulose.